Intermittent (once daily) parathyroid hormone (iPTH) administration increases bone mass by stimulating bone formation and is used clinically to treat osteoporosis. In contrast, continuous elevation of PTH (cPTH) results in severe parathyroid bone disease, a serious medical condition characterized by peritrabecular bone marrow fibrosis (osteitis fibrosa), reduced bone quality, increased bone turnover, and increased fracture risk. The mechanisms for these opposing biological effects are not well understood. Our findings in humans and rats provide strong evidence that an unexpected bystander, the mast cell, is essential for the differential skeletal responses to iPTH and cPTH. In further support of this hypothesis, mature mast cells are absent in bone marrow of mice and, compared to humans or rats, mice exhibit a greatly attenuated skeletal response to iPTH and cPTH. Based on extensive preliminary data, we hypothesize that high levels of PTH promote migration of resident mast cells to bone surfaces, where the mast cells express growth factors (platelet-derived growth factor) and chemokine activators (lysyl oxidase) that induce differentiation and migration of osteoblast lineage cells to bone. By inducing transient increases in mast cell-produced growth factors, iPTH promotes recruitment and differentiation of preosteoblastic fibroblasts to osteoblasts, leading to increased bone formation. In contrast, continuous overexpression of growth factors by mast cells in response to cPTH stimulates fibroblast migration to bone surfaces but prevents progression of fibroblasts to osteoblasts, leading to marrow fibrosis and increased bone turnover. Although a substantial body of evidence supports this scenario, further progress in deciphering the underlying molecular and cellular mechanisms is impeded by lack of appropriate animal models to evaluate PTH action in the presence and absence of mast cells. The objective of this R21 proposal is to establish a novel mouse model to assess the precise role of mast cells in the differential skeletal response to iPTH and cPTH. This will be accomplished by performing analyses on bone specimens obtained from mice engrafted with mast cells, which home to bone marrow. The following two Specific Aims are proposed: Aim 1: Define the correlative relationships among presence of mast cells, bone formation, and bone resorption in adult mice treated with iPTH. Aim 2: Define the correlative relationships among presence of mast cells, bone formation, bone resorption, and extent of bone marrow fibrosis in adult mice treated with cPTH. At the completion of this project, we expect to have established that the presence of mast cells within the bone marrow environment amplifies the magnitude of the bone anabolic response to iPTH and is required for cPTH-induced parathyroid bone disease. The primary positive impact of our anticipated findings is validation of a mouse model that will be used to further define the mechanisms by which mast cells mediate the pharmacological/pathological actions of PTH on bone.